erlang-dns

Erlang/OTP DNS server

erlang-dns is an authorative non recursive DNS server I am writing for my site http://www.domain-name-registration.co.za.

The idea is to have a simple DNS server that can be configured with arbitrary Zones via simple Erlang modules. It's up to you whether the Zone is defined in some file or a DB for example. As long as the module returns a zone as a list of resource records erlang-dns can serve it up.

In addition, erlang-dns supports custom extensions to enhance or entirely change the server's behaviour.

Releases

• 2014-01-11 v0.1-alpha - First version that conforms to RFC1034 examples

Installation

On Ubuntu please install Erlang base and source with sudo apt install erlang-base erlang-src. The sources are required since this server depends on kernel/src/inet_dns.hrl.

Run the end to end tests with make e2e to confirm that everything works.

Example

-module(my_zone_provider).

-export([get_zone/1]).

-include_lib("kernel/src/inet_dns.hrl").

get_zone(_Args) ->
    %% Fetch the Zone from a file, the DB, ...
    %% Here we just hardcode it
    {ok, [                                     
        #dns_rr{domain="bot.co.za", type=soa, data={   
            "ns1.bot.co.za",                         
            "hc.vst.io",              
            870611,          %serial
            1800,            %refresh every 30 min 
            300,             %retry every 5 min
            604800,          %expire after a week
            86400            %minimum of a day
            }
        },
        #dns_rr{domain="bot.co.za", type=ns, data="ns1.bot.co.za"},
        #dns_rr{domain="bot.co.za", type=ns, data="ns2.bot.co.za"},
        #dns_rr{domain="www.bot.co.za", type=cname, data="bot.co.za"},
        #dns_rr{domain="bot.co.za", type=a, data={127,0,0,1}}
    ]}.

Then, bring up the server with erl -pa ./ebin -s edns and register your zone provider.

edns:register_zone_provider("bot.co.za", {my_zone_provider, get_zone, []}).

If you change and recompile my_zone_provider you can flush the zone with

edns:flush("bot.co.za").

without restarting the server.

Also, please have a look at the-end-to end test scenario for an example of how to setup the included src/simple_backend.erl zone provider module.

The simple_backend consists essentially of only the following two lines:

get_zone(Zone) ->
    {ok, Zone}.

Instead of writing the my_zone_provider above, you could have achieved the same result with:

Eshell V5.10.2  (abort with ^G)
1> edns:register_zone_provider("bot.co.za", {simple_backend, get_zone, [
        #dns_rr{domain="bot.co.za", type=soa, data={   
            "ns1.bot.co.za",                         
            "hc.vst.io",              
            870611,          %serial
            1800,            %refresh every 30 min 
            300,             %retry every 5 min
            604800,          %expire after a week
            86400            %minimum of a day
            }
        },
        #dns_rr{domain="bot.co.za", type=ns, data="ns1.bot.co.za"},
        #dns_rr{domain="bot.co.za", type=ns, data="ns2.bot.co.za"},
        #dns_rr{domain="www.bot.co.za", type=cname, data="bot.co.za"},
        #dns_rr{domain="bot.co.za", type=a, data={127,0,0,1}}
    ]}).

The example above requires the record defintion for #dns_rr to be loaded. This can be done with rr("/usr/lib/erlang/lib/kernel-6.5.1/src/inet_dns.hrl")..

Port

Note that the server listens by default on port 1053. You may wish to forward 53 to it using socat.

sudo socat UDP4-RECVFROM:53,fork UDP4-SENDTO:localhost:1053

Extensions

Extensions live in the priv/extensions folder. Have a look at the simple_stats extension that counts the number of DNS requests.

Extensions can consist of custom resolvers and OTP servers/supervisors.

Custom Resolvers

By default, queries are processed by ed_query_resolver, however you can implement a custom resolver to augment or swap out ed_query_resolver entirely.

edns.app.src specifies what resolver(s) to use:

{env, [
     {port, 1051},
     {resolvers, [ed_query_resolver, simple_stats_resolver]}
   ]}

ed_udp_handler_server foldls over all resolvers to arrive at a DNS query response.

The simple_stats_resolver is a passthrough resolver that does not modify the repsonse. All it does is to notify simple_stats_server that another query has arrived.

Another example usecase of a custom resolver is one that looks up the synopsis of a Wikipedia article and returns it in a TXT record.

Custom OTP servers/supervisors

Modules in the extensions/... directory that implement either the gen_server or supervisor behaviour are automatically started and supervised by ed_extension_sup. This is useful if your custom resolvers need to maintain state.

Tests

To run the end to end test execute make e2e.

System Architecture

Please see the wiki https://github.com/hcvst/erlang-dns/wiki/Architecture

TODO

• sanity-check the zone provided by a callback module
• use inet_dns ADT helper functions in the resolver rather than pattern matching.

Notes

Section 4.3.2 Algorithm in http://tools.ietf.org/html/rfc1034